Telemetry systems are known for receiving alarm surveillance data and for supplying control data to remote equipment. One such telemetry system which has been widely used employs the Telemetry Byte Oriented Serial (TBOS) protocol. TBOS is typically used to monitor and control remote equipment on a point-to-point basis. A telemetry remote unit is employed to consolidate up to eight TBOS communication links into one serial control communication link for communicating to a maintenance center. In prior arrangements, a separate TBOS communication link had been required to monitor and control each individual equipment unit. If eight units were to be monitored and controlled, all eight TBOS communication links were required. A feature of TBOS arrangements is that if an equipment unit does not respond to a predetermined number of request messages from a telemetry remote unit, the TBOS communication link is considered to have failed and will be taken out of service.
More recently, attempts have been made to monitor and control more than one equipment unit using only one TBOS communication link. One such arrangement includes an equipment bay having a plurality of shelves with an equipment unit on each shelf. A TBOS communication link from a telemetry remote unit is connected to an equipment unit on the bottom shelf in the equipment bay and the data being received and transmitted is communicated to and from, respectively, the other equipment units in the bay in a cascaded fashion. This prior arrangement operated satisfactorily when the equipment units on all the shelves of the bay remained equipped and operated properly. However, if there was a shelf power failure or a shelf controller was removed, the telemetry remote unit would stop receiving response messages from the equipment unit on that shelf. The equipment unit on the shelf immediately below it would respond for the failed equipment unit, but all equipment units on shelves above the shelf including the failed equipment unit would cease to be monitored and controlled. Such occurrences are not acceptable.